The present invention relates a fiber-reinforced plastic pipe the ends of which are fixed to couplers.
A typical method for manufacturing a fiber-reinforced plastic (FRP pipe) is a filament winding method. In this method, fibers are impregnated with resin and are wound about a mandrel. Thereafter, the resin is hardened. The strength of a pipe made by the filament winding method greatly depends on the winding angle and the arrangement of the wound fibers. The winding angle of a fiber refers to the angle defined by the fiber and the mandrel. If the winding angle is set equal to or smaller than forty-five degrees, the winding angle cannot be easily maintained and the fastening force of a fiber is weak. Also, the pitch between the turns of the fiber cannot be maintained at a predetermined value.
To manufacture a rigid FRP pipe, bundles of fine fibers are used. If the winding angle of a bundle is small, the bundle is relatively loosely wound about a mandrel. As a result, it is difficult to form a cylinder of a uniform thickness with the bundle. Also, air or too much resin may be trapped between the fibers. Further, to increase the rigidity of an FRP pipe, the ratio of the volume of the fibers to the entire volume of the FRP pipe, or the volume fraction, herein after referred to Vf, of the fibers, must be increased.
Japanese Unexamined Patent Publication No. 8-290487 discloses a filament winding method. In this method, a helical layer is formed about a mandrel. The helical layer includes a bundle of reinforcing fibers. The bundle is wound about the mandrel at a relatively small winding angle. A hoop layer is formed about the helical layer. The hoop layer includes a bundle of reinforcing fibers. The bundle of the hoop layer is wound about the helical layer at a relatively great winding angle. The fastening force of the hoop layer is greater than that of the helical layer. When the hoop layer is wound about the helical layer, voids in the helical layer are pushed to the exterior. Also, excessive resin in the reinforcing fibers in the helical layer is squeezed out, which increases the Vf.
A propeller shaft made of an FRP pipe having yokes attached to the ends is known. For example, Japanese Unexamined Patent Publication No. 8-108495 discloses a propeller shaft 50, one end of which is shown in FIG. 6. The shaft 50 includes an FRP pipe 51 and metal yokes 52 attached to the ends of the pipe 51. The pipe 51 includes a helical layer 53 and hoop layers 54. The helical layer 53 extends in the longitudinal direction of the pipe 51. Reinforcing fibers of the helical layer 53 are angled by xc2x1five to thirty degrees relative to the axis of the pipe 51. The hoop layers 54 are located radially inside the helical layer 53 at the ends of the pipe 51. Reinforcing fibers of the hoop layers 54 are angled by xc2x1eighty to ninety degrees relative to the axis of the pipe 51. The hoop layers 54 are strong enough to bear a force by which the yokes 52 are force inserted into the ends of the pipe 51.
To increase the rigidity of an FRP pipe, the Vf is preferably raised. Also, increasing the Vf raises the coefficient of elasticity of the shaft, which increases the resonant frequency of the pipe. Therefore, an FRP pipe having a high Vf is suitable for a propeller shaft.
However, a higher Vf results in less resin between the helical layer 53 and the hoop layer 54, which is likely to cause delamination. Particularly, stress generated in the helical layer 53 is different from that in the hoop layer 54, which further promotes delamination. Therefore, an increased Vf reduces the durability of the ends of the FRP pipe 51.
Accordingly, it is an objective of the present invention to provide a durable fiber-reinforced plastic pipe that has improved rigidity and an improved coefficient of elasticity.
To achieve the foregoing and other objects and in accordance with the purpose of the present invention, a fiber-reinforced plastic pipe having a joint section formed in one end is provided. The joint portion is coupled to another member. The pipe includes a helical layer formed with a fiber. The fiber is wound at a winding angle that is equal to or less than forty-five degrees. The fiber volume fraction of the joint portion is lower than the fiber volume fraction of the remainder of the pipe.
The present invention may also be embodied in a method for manufacturing a fiber-reinforced plastic pipe having a joint section formed in one end and a body section joined to the joint section. The joint section is adapted to fit another member. The method includes winding a fiber impregnated with resin about a mandrel to form a helical layer, the winding angle of the fiber being equal to or less than forty-five degrees relative to the axis of the mandrel; winding an organic fiber about the helical layer at a winding angle that is substantially ninety degrees relative to the axis of the mandrel; and tensioning the fiber while the fiber is being wound such that the fiber volume fraction of the joint section is lower than the fiber volume fraction of the body section of the pipe.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.